On April 15, more than 300 teenage schoolgirls were abducted from the Chibok Government Girls Secondary School in northeast Nigeria. At least 53 girls are known to have escaped.

Who did it?

Boko Haram - which means "western education is a sin" - is an Islamic militant group in Nigeria. The group's leader, Abubakar Shekau, is taking responsibility for the mass abduction, according to a video obtained by Agence France-Presse.

In the video, Shekau described the girls as slaves and said, "By Allah, I will sell them in the marketplace."

Unconfirmed reports last week said the girls were being sold as brides for $12 each.

The goal of Boko Haram is to"destroy the secular Nigerian state" and replace it with an Islamic state, said John Campbell, former U.S. ambassador to Nigeria.

According to one of the escaped girls, the students heard gunshots from a nearby town. Uniformed men arrived saying they were with the Nigerian military and told the girls to go outside. The men were actually kidnappers.

Where are the girls now?

Authorities still don't know.

What's the Nigerian government's response?

The Nigerian government's response has been "so little, so late," said Bronwyn Bruton, deputy director of the Africa Center at Atlantic Council, an international affairs think tank.

Nigerian President Goodluck Jonathan was silent on the kidnapping for three weeks, only making a public announcement about the situation on Sunday.

Little details have been released on the government's investigation, Bruton said.

After the release of the Boko Haram video, the Nigerian government said it would not be deterred in rescuing the girls. But a government spokesman also criticized the girls' parents for not fully cooperating with police, according to comments by Doyin Okupe, spokesman for President Jonathan, to CNN.

In recent years, the government's brutal crackdown on Boko Haram members has also resulted in a "huge number" of civilian casualties, and locals do not trust the government or the military, Bruton said.

"When it's such a communication vacuum between government and locals, it really is a problem," she said.

Another possible reason for the government's lack of action is that Boko Haram is well-armed and knows the local terrain better than the military, said Jacob Zenn, an Africa analyst for the Jamestown Foundation, a Washington, D.C. research institute focused on international terrorism.

So far, the Nigerian government has "given a lot of rhetoric, but it hasn't been held up by action, and many are losing hope of getting their girls back," Zenn said.

Since the kidnapping took place, the U.S. State Department has been briefed "several times" about the situation, and the U.S. government had offered counterterrorism help. Also, six senators have introduced a resolution calling for the immediate return of the girls.

What's the public's response?

The continued search for the girls has increased the calls for more action from the Nigerian government. More than 280,000 have signed a Change.org petition calling for the girls to be returned. On Twitter, users are using the hashtag #BringBackOurGirls, which has been tweeted nearly 1 million times, Mashable reports.